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When main ducts are split into branches the sound power level - Lw - is reduced according the size of the branch, or amount of air to the branch.

The attenuation can be calculated as
dLw = 10 log(Ab/Am) (1)
where
dLw = attenuation (dB)
Ab = branch area (m2, ft2)
Am = main duct area (m2, ft2)
The attenuation is indicated in the diagram below:

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